234 



HIGH SCHOOL ZOOLOGY. 



ference is in the organization, of the head. Not only are the chief 

 nerve-centres and the sense-organs aggregated here, but there is 

 developed a complicated meclianism in connection with the 

 mouth, consisting of horny jaws and a lingual ribbon or radula, 

 tlie surface of which is beset with teeth like a rasp or file, and 

 which can be everted by special muscles. 



20. Three-fifths of the Gastropods ai'e adapted for breathing 

 air, the mantle-cavity being altered into a lung and the gills be- 

 ing rudimentary (cf. VII, 12); they form an important order Pul- 

 monata, a key to the structure of which is furnished by the pond- 

 snail figui-ed above. It belongs to a sub-order, the men.bers of 

 which ( Basommatopliora) have the eyes at the bases of the 

 tentacles, and possess thin shells, which may be spiral like Lim- 

 ncea, or spiral and dexiotropous like PJujsa, or coiled in one 



plane like Planorhis, or 

 simply conical like An- 

 cylus (Fig. 159). More 

 numerous, however, are 

 the land-snails and slugs 

 which cai-ry the eyes at 

 the tips of the tentacles 

 (^Stylominatoi^hora), and 

 which, include shelled 

 forms like HellXjZonites, 



-Shells of fresh water Gasteropoda. 



Fls- 159.- 



4 AnciiUin; b, ria,i<H-l>is. Prosobranchs, -6, / o^M- ^.,(ccinea and fomiS With 

 ,;mrt, with the operculum in the aperture ; <,Gmuo. • _l i 



basiii. a rudimentary internal 



shell like Liniax. 



21. A second order of Gastropods— ProsobrancMata— includes, for the 

 most part, marine forms, differing from the rulmonata in possessing a 

 trill in the mantle-cavity, and, usually, an operculum carried on the foot for 

 closing the aperture of the shell. The ordinal name is derived from the 

 fact that the respiratory organ is situated in front of the heart, as it is in 

 the Puhnonata. Great variety of colour and form characterizes the shells 

 of ths Order, the Chitons, e.g., having a shell formed of eight transverse 

 pieces, the Limpets (Patella), a simple conical shell, while endless va- 



